Pedicle screws are basically used for dorsal stabilization of the spine by means of transpedicular screwing in the event of fractures, tumors, inflammations, deformities and degenerative instabilities. Pedicle screws are placed in the pedicles of adjacent vertebrae, whereupon an angularly stable connection is created between the axially superimposed pedicle screws and an axially extending longitudinal beam or bridge. The pedicle screws and longitudinal beams form a vertebral stabilization system.
For this purpose, a pedicle screw usually has an axial, shank-like external thread section which is adjoined by a so-called tulip on the screw head side. This tulip forms a U-shaped, longitudinally slotted/tunneled holding sleeve with an internal thread, whereby each of the two radially opposite longitudinal slots defines a slot gap of predetermined gap width. The longitudinal beam/rod is inserted transversely in the parallel extending longitudinal slots and fastened by means of a locking element, for example in the form of a grub screw, threaded nut or set screw, which is screwed into the internal thread.
In general, two basic types of pedicle screws are distinguished, namely uniaxial and polyaxial pedicle screws. In the case of a uniaxial pedicle screw, the external thread section/shank and the tulip/holding sleeve are formed in one piece. A polyaxial pedicle screw, however, has an external thread shank manufactured as a separate component with a mostly spherical or (semi-) spherical screw head, with the holding sleeve/tulip gripping it in a relatively pivotal manner and simultaneously reaching behind the transition area between head and shank. In this way, after lowering the external thread shank into the pedicle channel of a vertebra, the holding sleeve/tulip can be swiveled and/or twisted relative thereto in order to obtain a desired position and orientation substantially independent of the orientation of the shank. The undercut prevents the holding sleeve/tulip from being removed from the shank head. Next, the holding sleeve/tulip is positionally fixed on the screw head by means of the locking element/set screw with an intermediate web (single-screw principle) or by means of an additional screw/screw nut (multiple-screw principle).
Pedicle screws are inserted by a surgeon into the pedicle canal of a vertebra or anchored by screwing. In doing so, the surgeon aligns the screws based on the orientation of the pedicle canal. When the screws are set, the aforementioned longitudinal beam or bar having the correct length is selected and, if necessary, adapted in its longitudinal curvature to the pedicle screws and their respective positions. The longitudinal beam is to be placed in the tulips of the pedicle screws.
The fact that the holding sleeve of a polyaxial pedicle screw is movably supported relative to the threaded shank makes it easier for the surgeon to insert the longitudinal beam laterally into the holding sleeve. If the surgeon is satisfied with the fit of the longitudinal beam and the holding sleeve, he locks the pedicle screw by means of the locking element, preferably the grub screw (in principle also called a set screw of any configuration). In the case of the above-mentioned single-screw principle, the polyaxial locking (positional fixation of the holding sleeve with respect to the shank) and the longitudinal beam clamping are realized in a single assembly step during tightening the one set screw.
Due to the mobility (joint) between the screw shank and the holding sleeve during the implantation process, it is generally not possible with polyaxial screws to transfer forces from the holding sleeve into the vertebra in order to be able to manipulate it, for example. However, this is particularly necessary for repositioning maneuvers in the case of fractures or spondylolistheses and partly for compressions or distractions. Therefore, uniaxial pedicle screws are used for this purpose, in which the holding sleeve is rigid with respect to the screw shank. However, these uniaxial pedicle screws have the disadvantage that it is difficult to insert the longitudinal beam laterally into the tulip/holding sleeve of several uniaxial pedicle screws.
With many pedicle screws as described above, the fixing means/locking elements (screws) are essentially self-locking to avoid the risk of unintentional loosening of the longitudinal beams from the pedicle screws after implantation. In addition, the fixation forces between the pedicle screw and longitudinal beam are considerable, as the entire system must withstand high loads without the set positional relationship between the threaded shank, holding sleeve and longitudinal beam being allowed to change. This means that the clamping forces between the shank head and the holding sleeve are also very high, so that the frictional connection forming between them is often not released even when the fixing means is released. However, these necessities also cause problems during the implantation process.
As long as the holding sleeve is not fastened relative to the shank head, e.g. firmly connected to the shank head by a frictional connection, no adjustment force can be transmitted to the vertebra by it. In the case of a polyaxial pedicle screw according to the single-screw principle, a longitudinal beam would first have to be inserted and then the locking element, preferably the grub screw, tightened in order to finally be able to apply an adjusting force to the vertebra. However, this would not make technical sense.
Once an operator has tightened the locking element/set screw with force and a subsequent adjustment of the vertebral position may be necessary, the positional relationship thus determined cannot be changed again, or only with great effort. In other words, the surgeon would have to loosen the grub screw(s) tightened with great force against their self-locking effect without loosening or even breaking out the external thread section which is already anchored in the vertebra. Even if the grub screw can be loosened without any problems, a self-locking frictional connection may have developed between the shank head and the holding sleeve. Even if the grub screw is loosened, this frictional connection could only be released with considerable force (knocks on the holding sleeve, etc.). Furthermore, subsequent loosening of the locking element may impair its self-locking effect, so that the functionality of the pedicle screw is no longer guaranteed.
A pedicle screw is known from WO 2013/034351 A1 in which the polyaxiality can be provisionally fixed so that the polyaxiality and the web/longitudinal beam can be locked independently at least temporarily, i.e. only the polyaxiality is fixed without the web being fixed. This publication also discloses a medical device or auxiliary means for provisionally fixing the polyaxiality of a pedicle screw. The pedicle screw essentially consists of a screw shank with a shank head, which is surrounded by a holding sleeve. This holding sleeve supports a plunger that can be pretensioned against the shank head by a locking element. The instrument has a first component in the form of a pressure tappet in order to get into pressing engagement with points of attack provided directly on the plunger. It also has a second component in the form of a sleeve-shaped traction component which is movable relative to the first component and has a latching means which can be brought into pulling engagement with the holding sleeve. The disadvantage of this instrument is that the traction component must be expanded in the area of the pedicle screw's holding sleeve in order to uncouple the traction component. Under certain circumstances, however, this is hardly possible, as bony structures may stand in the way. In addition, the possibility of accidental disconnection of the pedicle screw's holding sleeve cannot be reliably excluded with this instrument.
From US 2014/0277137 A1, a medical instrument is known for provisionally fixing the polyaxiality of a polyaxial pedicle screw, in which an outer component is designed to come into engagement with a holding sleeve of the polyaxial pedicle screw. An inner component received in the outer component is movable relative to the outer component in the axial direction so that the inner component can exert a proximal force on a compression element received in a proximal area of the holding sleeve and thus fix the holding sleeve in position. With this instrument, it is a disadvantage that the locking element/set screw cannot be screwed into the dedicated thread of the holding sleeve in the state when temporarily fastened (with Poly-Lock activated). Furthermore, the small undercut of the holding sleeve can only transmit low fixing forces.
At this point it should be pointed out that the term “provisionally fixing or fastening”, especially with regard to the invention described below, is not necessarily meant to mean only easy application/clamping of the shank head, but in particular such a locking measure which is used temporarily and whose achievable locking properties such as the clamping force on the shank head, etc. definitely correspond to, at least come close to or even exceed the permanent locking. In other words, the “provisional” locking should be dimensioned such that it can also cause the holding sleeve to jam on the shank head in accordance with the permanent locking element. In this case, it may be possible to subsequently loosen the frictional connection in accordance with the state of the art by applying the appropriate force, but it is possible to readjust the vertebrae before fixing the longitudinal beam. It is therefore desirable to dimension the “provisional” locking in such a way that sufficient adjustment forces can be transferred to the vertebra by means of the polyaxial pedicle screw, whereby the frictional connection achieved between the shank head and the holding sleeve can be released again.